In the various methods for the metallurgical treatment of liquid metal or steel in a vacuum, tubular blow lances and/or burner lances are introduced into a vacuum chamber from the outside, vertically or diagonally from above, using a vacuum-sealed duct. The purpose of these lances is to introduce gases such as oxygen, argon, nitrogen or solids with a carrier gas into the steel melt located in the vacuum chamber.
Lances of this type are generally connected to a guide slide which can be displaced by a motor and by means of which the lance can be moved into the required operational position. A gland of a known, conventional construction, encompassing the lance, acts as a vacuum seal.
The position and alignment of the gland should be axially parallel to the slide path of the guide slide so as to allow undisturbed feeding-in and retraction of the lance. In reality there are deviations here, caused on the one hand by measurement and manufacturing tolerances and on the other hand by thermal distortion of the vacuum chamber cover, and so the lance axis does not always extend parallel to the slide path. Consequently, there are often high frictional forces between the lance surface and the gland, and these can cause the gland to become leaky or the slide drive to malfunction.
To avoid these serious drawbacks, DE 195 18 361 C1 suggests using a gland with a thin-walled metal compensator to thus compensate for deviations in the parallelism of the lance axis and the slide path. The lance duct with compensator according to the prior art disclosed in the above-mentioned patent is constructed in such a way that the same pressure prevails inside the compensator as in the treatment chamber, i.e. a vacuum prevails in the case of vacuum treatment, and the atmospheric external pressure acts on the outer wall of the compensator. The atmospheric pressure of approx. 3000 kg acting on the compensator from outside braces or deforms the compensator, greatly limiting or impeding the required flexibility thereof. Moreover, the high forces acting on the compensator in combination with axial and lateral deviations in the movement path of the lance can lead to the formation of ruptures in the compensator and thus to the failure thereof. Although the solution proposed in DE 195 18 361 C1 allows an improvement to be achieved as regards axis parallelism, it is nevertheless, as described above, susceptible to failure and thus not always reliable.